Distribution of Sensory Impairment in Distal Ulnar Sensory Branch Injury at the Wrist
Injury to the distal ulnar sensory branch at the wrist produces sensory loss primarily affecting the little finger and the ulnar (medial) half of the ring finger on the palmar surface, with additional involvement of the ulnar side of the hand and dorsal aspects of these digits.
Palmar (Volar) Sensory Distribution
The superficial sensory branch of the ulnar nerve at the wrist divides approximately 0.9 cm distal to the pisiform into superficial and deep divisions 1. Sensory impairment from a distal ulnar sensory branch lesion at the wrist manifests as:
- Complete sensory loss over the entire little finger (palmar surface) - this is the most consistently affected area 2
- Sensory loss over the ulnar half of the ring finger (palmar surface) 3
- Sensory loss over the hypothenar eminence and ulnar border of the palm 2
The palmar ulnar cutaneous nerve (PUCN), which originates approximately 11.9 cm proximal to the pisiform, supplies sensation to the ulnar palm and may be spared in very distal wrist injuries 1.
Dorsal Sensory Distribution
The dorsal ulnar cutaneous nerve (DUCN) branches approximately 7.0 cm proximal to the pisiform and typically passes around the ulnar styloid 1. However, its terminal branches contribute to wrist-level sensory distribution:
- Dorsal aspect of the little finger - consistently affected, with SNAPs always recordable from this digit 3
- Dorsal aspect of the ring finger (proximal portion) - predominantly supplied by the DUCN 3
- Ulnar dorsal hand - variable involvement depending on exact lesion location 1
The contribution of the DUCN decreases progressively from the little finger (always present) to the middle finger (rarely present), with the ring finger showing intermediate involvement 3.
Key Clinical Examination Findings
Sensory testing should focus on the little finger as the most reliable indicator of ulnar sensory nerve dysfunction at the wrist 2. Examination reveals:
- Monofilament testing abnormalities predominantly over the little finger and ulnar hand 2
- Preserved sensation over the dorsal first web space (radial nerve territory) 3
- Potential sparing of the ulnar palm if the PUCN branches proximal to the injury site 1
Important Anatomical Variations
Be aware of these common pitfalls:
- Median-ulnar communications can occur, where anomalous median nerve branches supply the little finger, potentially confusing the clinical picture 4
- The middle finger dorsum is predominantly innervated by the median nerve, not the ulnar nerve as traditionally taught, so its involvement suggests a more proximal or different lesion 3
- Motor function is typically preserved in pure distal sensory branch injuries at the wrist, distinguishing this from more proximal ulnar nerve lesions 2
Electrodiagnostic Confirmation
Nerve conduction studies can localize the lesion by comparing wrist-to-finger versus palm-to-finger segments:
- Stimulation 7 cm proximal to the active electrode (palm segment) versus 14 cm (wrist segment) helps identify focal slowing 5
- Normal amplitude ratio between palm-to-finger and wrist-to-finger segments is 1.4 to 0.76 5
- Abnormal SNAP amplitude ratios or substantial slowing across the wrist confirms the diagnosis 5
The cross-sectional area of the superficial sensory branch measures approximately 3.9 mm² at the wrist level, making it identifiable on high-frequency ultrasound 1.